In recent years, there has been an increased demand in commercial and military applications for improved optical filter devices. For instance, in the area of night imaging systems, there has been an increased need felt to develop a filter assembly to enable simultaneous viewing of infrared images, and other select wavelengths of light, while discriminating against undesired optical radiation. As used herein, the term "night imaging system" refers to an optical system capable of intensifying images viewed under low light levels, e.g. nighttime conditions. Night imaging systems are popularly employed in aviation applications. An example of a night imaging system would be the ANVIS system (Aviator's Night Vision Imaging System), which is produced by Hughes Optical Products, Inc. The ANVIS system is a helmet-mounted unity power image intensified binocular that enhances vision under low light level conditions. A discussion of the ANVIS system can be found in Efkeman, Jenkins, Development of an Aviator's Night Vision Imaging System (ANVIS), presented July 28-Aug. 1, 1988, SPIE International Technical Symposium and Exhibit.
When used in many aircraft applications, particularly military aircraft, unfiltered night imaging systems such as an unfiltered ANVIS system, suffer some disadvantages. For instance, the panel lights in many cockpits tend to overdrive the image intensifier in the imaging system. Design considerations for producing filter systems, such as those having a maximum visibility of about 530 nanometers (nm), for night imaging systems previously have been discussed. See e.g., B.D. McMains, Recommendations for Color Limitation of Illuminated Devices used in connection with AN/AVS-6 Night Vision Goggles, presented Oct. 19, 1983, SAE A-20A subcommittee - meeting No. 52.
It has been proposed as a solution to some of the problems encountered while using night imaging systems to implement a filter, such as a standard minus-blue filter, into a night imaging system. Minus-blue filters typically are suitable for filtering cockpit lighting, while still failing to substantially interfere with the viewing of outside imagery.
Unfortunately, the use of a minus-blue filter tends to inhibit viewing of images from cathode ray tubes (CRT), such as those found in head-up display units. This is largely due to a high attenuation of green light, emitted from the CRT, caused by the minus-blue filter. The typical attenuation is of a magnitude that, when using minus-blue filters, the CRT has to be employed at such high intensity levels that a useful image is not easily obtained.
One proposed solution to the problems encountered while using night imaging systems has been to implement a narrow band phosphor filter, and/or narrow band transmitting faceplate filter, to the CRT display. A complementary narrow band reflection filter can then be placed over the objective lens of an image intensifying goggle. See, L.C. Taylor, Compatibility of Night Vision Goggles with CRT Displays in a Helicopter Cockpit. Unfortunately, this approach tends to reduce the intensity of the CRT display to a relatively undesirable level for many applications.
It has further been proposed that a partially filtered imaging system be employed to reduce light transmission of a first wavelength, yet pass through light of a second wavelength. For instance, it has been suggested that a partially filtered imaging system be employed to reduce glare from cockpit lights, yet pass through infrared radiation and green light from a CRT, to permit viewing of a head-up display image.
One such imaging system might employ an input aperture having all but a small area which is covered by a filter. The filter is typically employed to reduce glare. The small unfiltered area, however, would pass light from the head-up display image.
The use of filter systems such as the Kodak Wratten filter system is known for applications requiring selective filtration. Unfortunately, the use of that system tends to be impractical for many applications requiring very sharp cutoffs and narrow band widths.
Finally, narrow band filters used alone or in combination with other such filters have been proposed to solve one or more of the above problems. A typical filter would be one such as a Schott BG-7B filter which is a broadband blue green transmitting filter. That filter typically has about a 60 nm transmission band. Unfortunately, because the head-up display transmits a more narrow band, i.e. about 20 nm, the relatively broad band of filters like the Schott BG-7B renders it unsuitable for many applications. That is, broader transmission bands of the filter introduce greater amounts of undesired light.
The need for a system that selectively transmits predetermined wavelengths of light has further been discussed in K. Miller, Accurate Light Measurement in Aircraft Cockpits, Electrooptics, pp. 26-30, July 1983.